The present invention relates to an austenitic steel which exhibits not only improved corrosion resistance but also a satisfactory level of high-temperature strength. The austenitic steel of the present invention can exhibit improved high-temperature properties when used in boilers and chemical plant equipment operated at high temperatures.
In general, a satisfactory level of high-temperature strength, corrosion resistance, weldability, and other properties are required for materials which are used in a variety of boilers and chemical plant equipments which are operated at high temperatures. For these purposes, 18-8 type austenitic stainless steel has been widely used so far, since these properties are satisfied to a fair degree and it is not so expensive.
However, recently, a higher thermal efficiency has been required for these high-temperature applications and this higher efficiency has been achieved by higher operating temperatures. Accordingly, the materials used to manufacture these apparatuses have to resist more severe operating conditions. Thus, a higher level of performance has been required of the material, and this level of performance cannot be achieved using conventional 18-8 type austenitic stainless steel with respect to high-temperature strength and corrosion resistance.
It is generally well known in the art that an increase in the Cr content is effective to improve the corrosion resistance of a stainless steel. However, take SUS 310S (AISI 310S) steel as an example known at high Cr-steel, an increase in the Cr content does not always result in an increase in high-temperature strength, but sometimes adversely affects the high temperature strength.
Japanese Patent Application Laid-Open Specification No. 59-23855 discloses a steel with high-temperature strength containing carbide-forming elements. High-temperature properties of the steel disclosed therein are improved by finely dispersing carbides of Nb, Ti, Zr, and Ta, and the sulfur content should preferably be reduced to 0.010% or less in order to improve high-temperature strength and workability. However, the above reference does not mention about reduction in the amounts of impurities such as phosphorus and aluminum.